This invention relates to the field of automatic transmissions for motor vehicles. More particularly, the invention pertains to a kinematic arrangement of gearing, clutches, brakes, and the interconnections among them in such transmissions.
The number of forward drive speed ratios in automatic transmissions for motor vehicle has continued to increase due to the need to increase fuel economy and to avoid large steps between the speed ratios. Each known six-speed transmission in which no more than three gearsets and five friction elements (the clutches and brakes that control operation of the transmission) are employed has some drawbacks such as ratio progression, shared pinions, etc.
U.S. Pat. No. 5,106,352 describes a multi-speed automatic transmission for motor vehicles that includes two parallel gearsets providing two fixed speed ratios. The second speed ratio is higher than the first speed ratio. A first power path using the first fixed speed ratio includes a first control clutch and a second control clutch, and a second power path using the second fixed speed ratio includes a third control clutch. Several embodiments of the transmission include a double planetary gearset; other embodiments disclose a Ravigneaux gearset, a first control brake and a second control brake. Alternatively, the input shaft and output shaft are in alignment and one of the two speed ratios is a direct drive.
Experience in manufacturing Ravigneaux gearsets for automatic transmissions has shown that these gearsets tend to produce relatively high gear noise in comparison to simple planetary gearsets and Simpson gearsets. To minimize gear noise, extensive steps are taken during the manufacturing process to grind the gears. But these steps and the additional complexity associated with assembling Ravigneaux gearsets add cost to the transmission.
There is a need for a six-speed automatic transmission in which the planetary gearsets are entirely simple planetary gearsets rather than double pinion gearsets or shared pinion gearsets. Preferably such a transmission would require no more than five hydraulically-actuated friction clutches and brakes to produce step changes among the speed ratios.